When a printed circuit board is mounted with electronic components such as integrated circuits, condenser chips, resistor chips and the like, solder paste is applied by a screen printing apparatus to the board for soldering the electronic components to the electrodes of the patterned circuit thereon.
Generally, in a screen printing apparatus of such a kind, as shown in the Laid Open Japanese Patent Application No. Toku-Kai-Hei 4-65243, the printed circuit board is brought near the under side of the screen mask. A squeegee slides on the mask to apply solder paste onto predetermined points on the board through patterned holes located in the mask. A screen printing apparatus of the prior art, as shown in FIG. 9, comprises a screen mask 1 having a mask plate 2 applied to the under side of a holder 3 of a frame shape. A printed circuit board 4 is held on a table 5, having nuts 6a and 6b combined with vertical screws 7a and 7b. One screw 7a is rotatably driven by a motor 8. The screws 7a and 7b are provided with timing pulleys 9a and 9b, and a timing belt 10. The screw 7b has a bearing 11.
When motor 8 is energized or engaged, screw 7a rotates, and the rotation is transferred to another screw 7b via timing pulleys 9a, 9b and timing belt 10. By the simultaneous rotations of both screws 7a and 7b, table 5 and printed circuit board 4 ascend or descend with respect to mask plate 2. The apparatus is further provided with a squeegee 12 that slides on mask plate 2 to apply solder paste 13 on board 4.
By the rotation of motor 8, board 4 ascends, so that the upper surface of board 4 is placed in contact with the under surface of mask plate 2. If squeegee 12 is slid to the left on mask plate 2 by a driving means not shown in the figure, solder paste 13 is applied on the upper surface of board 4 through holes in plate 2. When motor 8 is rotated in the opposite direction, board 4 descends away from plate 2, thereby completing the solder paste application process.
In the past, in the application of solder paste 13, a small space (generally called "snap-off") had been preserved between mask 2 and board 4. However, as the leads of the electronic components have become arranged with an increasingly smaller and smaller pitch, the snap-off became smaller and smaller, eventually coming to null, in recent years. As a result, the application of solder paste 13 is performed with the under side of mask plate 2 touching the upper surface of board 4.
However, if the solder paste application is performed with the under side of mask plate 2 and the upper side of board 4 touching one another, then a problem occurs, namely, when board 4 is moved away from mask plate 2, the resultant shape of solder paste 13 on board 4 is undesirably deformed.
The reason for this deformation in shape can be explained with reference to FIGS. 10(a) to (c). FIG. 10(a) shows the state of solder paste 13 just after its application squeegee 12. The patterned holes of mask plate 2 are, as a result of sliding of squeegee 12, filled with solder paste 13, and by its adhesive power, the under side of mask plate 2 is adhered no the upper side of board 4.
If board 4 is lowered by the reverse rotation of motor 8, then mask plate 2 gradually bends downwards, because it undesirably adheres to board 4 by the adhesive power of solder paste 13 (FIG. 10(b)). When board 4 is lowered further, mask plate 2 abruptly separates from board 4, and returns to its horizontal state as shown by the solid line in FIG. 10(c) through its own elastic force. Mask plate 2 is made from a flexible thin plate of metal such as stainless steel.
In the process described above in which the printed circuit board 4 is lowered away from mask plate 2, the mask plate is abruptly separated by its own elastic force from board 4 when board 4 is lowered, thereby causing solder paste 13 in the patterned holes to become deformed or left in the holes.
Examples of solder paste applied by a conventional screen-printing apparatus are shown in FIG. 11, in which the configuration of solder paste 13b is a good one, and the configuration of solder paste 13c is an inferior or undesirable one deformed by the above described reason with the edge unnecessarily projecting. With such a deformed solder paste 13c, it is difficult to solder the lead of an electronic component thereto. Such deformation is notable as the pitch of the patterned holes becomes smaller, and is troublesome when electronic components with increasingly smaller lead-pitches are applied on a printed circuit board.
Likewise as in the apparatus described above in which screen mask 1 is stationary and printing board 4 descends, the same problem arises in an apparatus in which the mask descends to, or ascends from; the stationary board.
Thus, an object of the present invention is to provide a screen printing apparatus and method for applying solder paste on a printing board with a mask plate, whereby the applied solder paste does not become deformed when the mask and the board are separated.